The cell wall of the fungal pathogen C. neoformans is an essential organelle that incorporates chitin as a rigid, insoluble, polysaccharide polymer. De-acetylation converts chitin to chitosan, a more flexible and soluble polymer. While chitin is an essential component of fungal cell walls, the importance of chitosan to cell wall integrity is unclear. The overall goal of this project is to identify key steps in Cryptococcus cell wall biosynthesis that will make attractive targets for antifungal drugs. Our preliminary data indicate that chitosan is an abundant component of Cryptococcus cell walls, both during vegetative growth and during infection of mouse lungs. We have explored genes that contribute to chitosan production, and shown that only one of eight putative chitin synthases, and one of three putative chitin synthase regulators substantially impact chitosan levels. In addition, we have demonstrated that three of four potential polysaccharide deacetylases produce chitosan. The shared phenotypes among strains deleted of genes in the chitosan biosynthetic pathway, include defects in cell integrity and leaky melanin, and suggest that these phenotypes are the result of substantial reduction of chitosan. Data obtained from this project will establish the role of chitosan in C. neoformans cell wall integrity and virulence, how chitosan is attached to the cell wall, and identify critical steps in this aspect of cell wall biosynthesis that could serve as antifungal targets. This project has three specific aims. In the first, we will determine the composition of chitosan and how it is linked to the cell wall during growth in the mammalian host. In the second, we will test the roles of the three chitin deacetylases in chitosan biosynthesis, cell wall integrity and virulence. In the third aim, we will test our current model that one chitin synthase and its regulator together synthesize the chitin that is deacetylated by the three chitin deacetylases to generate chitosan and determine the interactions among the these proteins that are critical for chitosan production. The results of these studies should provide a better understanding of chitosan biosynthesis and function in C. neoformans, and determine if chitosan production would be a viable antifungal target for novel anti-cryptococcal therapy. Fungal infections have become more prevalent in recent years due to the increase in the immunocompromised patient population from AIDS, organ transplants and chemotherapies. Systemic fungal infections are serious health threats, and safe, highly effective antifungal therapies are not available. Biosynthesis of the fungal cell wall is an attractive target for antifungal therapies because the cell wall is an essential organelle that is not present in the human host, and this project will delineate the biosynthesis of the cell wall of a fungal pathogen, Cryptococcus neoformans, and determine if one of its components, chitosan, is a potential antifungal target.